The problem is that fuel efficiency is linked to the gross vehicle weight rating. In order to be able to sell a truck that can do truck things, manufactures are forced to make them massive. The popularity of Kei trucks in the US is evidence of the fact that people don't want or even need trucks this large. If you look at a 90s f250, they are smaller than the f150s of today.
I'm all for increasing fuel efficiency standards, but the economy needs work trucks to get work done. Not everyone wants or needs a massive f250 for the work they're doing. People don't need massive trucks to do landscaping or haul tools around. Fuel and emissions standards have killed the small trucks of the 80s and 90s.
At a certain point, it just comes down to how much energy you need to move something from point A to B. There is a minimum for that, it's why electric trucks lose massive amounts of range when towing. However, to meet fuel efficiency standards, tthey can't put bigger engine in a small chassis. This creates a problem where the amount of fuel to move from A to B is actually increased because inorder to make a platform capable of doing work, they have to make the trucks heavier.
The other ironic part is that they said small trucks don't meet satefy standards. So now we ended up with larger trucks that are more dangerous than the smaller trucks.
EDIT: and just to make a final point on vehicle size, people aren't going to buy a vehicle SMALLER than what they need. There is real demand for midsized SUVs and trucks, but when you price them closer to the big models people are going to go with the mentality of "well, the bigger one is only a few percent more expensive". And since they're almost always financed people usually only see it as an extra $50-100/m